A thin-wall metal tube having a predetermined length is manufactured by a process including: rounding a thin metal sheet having a larger length than a predetermined length in such a manner that two parallel edges thereof are brought close to each other; welding the two edges together to form a cylindrical tube; and cutting the tube to the predetermined length. One known tube cutting apparatus for use in cutting such a tube is configured to move a cutting tool abutting against the outer peripheral surface of the tube toward the inside of the tube radially of the tube (see Patent Document 1 for example).
In cutting a thin-wall tube having a low strength with such an apparatus configured to move the cutting tool from the outer peripheral surface toward the inside of the tube, however, the thin-wall tube is deformed radially, thus lowering the roundness of the resulting product. In view of this inconvenience, a tube cutting apparatus for use in cutting such a thin-wall tube has heretofore been proposed which is configured to position an outer blade into contact with the outer peripheral surface of the tube and move an inner blade positioned inside the thin-wall tube toward the outside of the tube while rotating the inner blade.
In this case, use of a hollow shaft member and an inclined cam can be conceived for applying the inner blade with a moving force acting radially of the tube while transmitting rotation to the inner blade. The hollow shaft member is adapted to be inserted into the tube coaxially with the tube, supports thereon the inner blade by means of a retainer member at a predetermined axial position for rotation and radial movement, and accommodates therein the inclined cam abutting against the retainer member for axial movement. As the inclined cam moves axially of the hollow shaft member with the hollow shaft member rotating, the position of contact between the retainer member and the inclined cam is displaced radially of the tube to cause the inner blade to move radially of the tube and rotate.
However, in order to move the inclined cam axially within the hollow shaft member while rotating the hollow shaft member on which an inner blade die is rotatably supported so as to be movable radially of the tube, the structure of the hollow shaft member to be inserted into the tube becomes complicated and upsized, resulting in an inconvenience that the apparatus cannot be used to cut thin-wall tubes having relatively small diameters of about 100 mm.
Use of a link mechanism pivotally supported in a plane perpendicular to the axial direction of the tube can be conceived as a component for causing the inner blade to move radially of the tube and rotate. In this case also, a structure for causing the link mechanism to operate becomes complicated and upsized and, hence, the apparatus is not suitable for cutting of thin-wall tubes having relatively small diameters.
In view of such inconveniences, the applicant of the instant application has proposed tube cutting apparatus and method which use a driving mechanism to be located outside the tube for moving the inner blade radially of the tube while rotating the inner blade (see Patent Document 2). The invention described in Patent Document 2 uses a planetary gear mechanism comprising a ring gear and a planetary gear as the driving mechanism for moving the inner blade radially of the tube while rotating the inner blade.
When the planetary gear is caused to revolve along the pitch circle of the ring gear, the planetary gear rotates about its central axis at a higher angular velocity than its revolution because of the difference in the number of teeth between the planetary gear and the ring gear. The planetary gear rotating about its central axis causes a second axis on which the inner blade is rotatably supported to rotate in a manner decentered to a first axis, so that the position of that portion of the outer peripheral surface of the inner blade rotatably supported on the second axis which is most distant from the center of the tube changes gradually.
As the position of that portion of the outer peripheral surface of the inner blade which is most distant from the center of the tube moves further away from the center of the tube, the outer peripheral surface of the inner blade comes into contact with the inner periphery of the tube and then becomes exposed exteriorly of the tube, thereby cutting the tube cooperatively with the outer blade.
When the planetary gear revolves NP times (NP=Z1/Z2 where Z1 represents the number of teeth of the ring gear and Z2 represents the difference in the number of teeth between the ring gear and the planetary gear), the inner blade is returned into its initial position. During the NP times revolutions of the planetary gear, the inner blade becomes partially exposed exteriorly of the tube at N positions along the periphery of the tube, with the result that the tube is cut throughout its entire circumference.